You should be able to find the MAC address using the router itself (my Verizon gateway and my old Linksys router can). As long as you are connected to the raspi, you should be able to run "arp -a" in cmd (on your computer) to get a list and just figure out which is the raspi.

I do not know why you are filtering MAC addresses, but if it is an attempt to secure your network from outsiders, it will not help. Anyone that can break your WPA2 key will know how to spoof their MAC address to mimic another device on the network; therefore bypassing a MAC filter.